Aquatic angiosperms at unusual depths in Shoal Lake, Manitoba-Ontario

Abstract

Field-Naturalist 100(3): 354-358. Shoal Lake (elevation 323 m) was found to contain extensive macrophyte communities growing at depths of 12-14 m and consisting of the angiosperms Elodea canadensis, Najas flexilis, Potamogeton foliosus, P. zosteriformis, Myriophyllum exalbescens, Ceratophyllum demersum, Megalodonta beckii, Zosterella dubia, Lemna trisulca and the aquatic moss Drepanocladus sp. Aside from one previous record for E. canadensis at higher altitude, the above angiosperm species have not been reported at such depths before. These communities were observed throughout two consecutive growing seasons. Plants at these depths received an estimated 0.5-1% of surface light. Besides light, other factors which may have allowed for the existence of these communities were warm summer temperatures in deep water and the presence of oxygen in the sediments during most of the ice-free season. Submerged aquatic vascular macrophytes are normally confined to shallower regions of lakes, where light penetration and temperature, in conjunction with other, less well understood factors are conducive to macrophyte growth and reproduction. Normally angiosperms do not occur at depths greater than 10 m in freshwater (Ruttner 1953; Sculthorpe 1967; Hutchinson 1975). Recently Sheldon and Boylen (1977) and Singer et al. (1983) have recorded angiosperms at greater depths but these records apply to acidified lakes of exceptional clarity (ultraoligotrophic), where very little phytoplankton is present to reduce light penetration to deeper strata. The present paper reports the existence of a diverse established macrophyte community at the unusual depths of 12-14 m in Shoal Lake, a large, nonacidified Precambrian Shield lake on the Manitoba-Ontario boundary. Shoal Lake has a mean elevation of 323 m above sea level and is the source of the water supply for the City of Winnipeg. Materials and Methods Sampling was conducted on a semi-monthly or monthly basis during the 1984-5 growing seasons. A set of samples was also collected under ice cover in late February 1985. Water samples were immediately placed on ice in a dark container and frozen within 7 hours. Deep water samples were obtained with a van Dorn sampler. Thawed samples were analyzed according to methods recommended by the American Public Health Association (1975, 1985). Nitrate was determined using an Orion lonalyzer model 407A with nitrate electrode. The pH was measured directly in the field with a portable pH meter. Photosynthetically active radiation (PAR) [400-700 nm] was measured using a Li-Cor Integrating Quantum Photometer equipped with an underwater quantum sensor. Temperature and dissolved oxygen were measured with a Yellow Springs Instruments Tele-thermometer and Oxygen Meter, respectively. Chlorophyll content of water was estimated by filtering 0.2 to 1 L of water in the field through Whatman No. 1 filter paper under suction from a hand vacuum pump. The filter was placed in a plastic bag on ice in darkness, and stored in the laboratory at-20° C in the dark. The filter was ground in a mortar with neutralized acid-washed silica sand and 15 mL 80% acetone containing 10 mM CaCOj (lime). The homogenate was centrifuged at low speed in a clinical centrifuge. Chlorophyll content of the supernatant was determined using the spectrophotometric method of Arnon(1949). Standing crop of the deep communities was estimated with the aid of SCUBA; all above-ground plant material was collected by hand within a randomly chosen circle of 20 m radius, and washed and dried at 70° C to constant weight. Results The unusually deep macrophyte communities were distributed over a large area, centered at approximately 49° 36'N, 95°04'W, near the entrance to Indian Bay of Shoal Lake. This location is henceforth referred to as the DW site. The ranges of chemical and physical parameters obtained at this site during the 1985 growing season are given in Table 1. Most water chemistry parameters showed a wide range of variation during the season, particularly for phosphorus and nitrate concentrations. Amounts of chlorophyll a in the water, indicative of phytoplank-354